Fillers are a widely-used component in most polymeric materials in which they perform a variety of functions, such as reinforcing physical strength properties, improving the processability of the polymer and reducing cost. Conventionally, carbon black is the most widely used filler in elastomer or rubber compositions, particularly those compositions used to form tires, because it provides the most improved strengthening and durability performance, while also improving road traction and processibility of the rubber material during manufacture.
However, while offering these advantages, the use of carbon black filler also increases the hysteresis of a rubber or elastomer compound, and may thus adversely affect the tire performance properties related to hysteresis when used as the material to form a tire tread. In the specific case of tires, the hysteresis of a tire tread affects traction (both wet and dry), rolling resistance (which relates to fuel efficiency), and heat generation (higher heat generated during service can reduce the durability of the tire tread). Because of this and other disadvantages associated with the use of carbon black, there is a need in the art for alternative filler materials for use in rubber and elastomer compositions.
One filler material that has been proposed as an alternative to carbon black is silica. When silica is combined with a coupling agent (coupling agents provide connective adhesion between the silica filler and the polymer chains of the rubber), it becomes coupled silica, which offers advantages compared to conventional fillers. For example, tire treads made from coupled silica-containing rubber have strength and durability properties comparable to tire treads made from carbon black-containing rubber. Indeed, the use of coupled silica filler provides certain benefits over conventional fillers, notably the use of coupled silica enhances the hysteresis-related properties such as increasing the wet traction, lowering the rolling resistance (and thereby improving fuel efficiency) and lowering the heat generated during service (and thereby improving the tire durability).
However, the use of coupled silica in rubber and elastomer compositions also presents certain disadvantages. Combining silica with coupling agents complicates the mixing process and manufacture. In particular, the manufacture is delayed because sufficient time must pass to allow for the completion of the reaction between the coupling agents, the polymer chains and the silica. Additionally, the reaction of silane coupling agents leads to ethanol emissions. Because ethanol emissions into the atmosphere are strictly regulated, manufacturing facilities using silane coupling agents must be equipped with equipment to control such emissions, further increasing costs.
Still other alternative fillers for rubber compositions have been proposed, but these alternative filler materials are not themselves without disadvantages. For example, while not an acceptable substitute on its own for carbon black, it has been suggested that a portion of carbon black filler in an elastomer or rubber composition could be replaced with barium sulfate. Barium sulfate's density would seem to make it an ideal filler (when blended with carbon black) in rubber compositions intended for use in making tires, because important tire performance characteristics such as rolling resistance and traction performance are related to the average density of the filler materials. However, because barium sulfate has neither sufficient surface area nor sufficient surface chemistry to interact with the rubber matrix, the barium sulfate fails to provide any reinforcement benefits, so that barium sulfate filler-containing rubber compositions do not provide sufficient abrasion resistance to be made into tire treads.
Given the foregoing, there is a continuing need for new filler materials for use in rubber compositions that can be formed into a tire with excellent performance characteristics. Such filler materials should be capable of being easily incorporated into a rubber composition without complicating or slowing down the manufacturing process. Additionally, such fillers should preferably reduce the amount of additional chemical compounds, like coupling agents, that are necessary for producing tires having satisfactory performance standards.